183 research outputs found
Dielectric properties and lattice dynamics of alpha-PbO2-type TiO2: The role of soft phonon modes in pressure-induced phase transition to baddeleyite-type TiO2
Dielectric tensor and lattice dynamics of alpha-PbO2-type TiO2 have been
investigated using the density functional perturbation theory, with a focus on
responses of the vibrational frequencies to pressure. The calculated Raman
spectra under different pressures are in good agreement with available
experimental results and the symmetry assignments of the Raman peaks of
alpha-PbO2-type TiO2 are given for the first time. In addition, we identified
two anomalously IR-active soft phonon modes, B1u and B3u, respectively, around
200 cm-1 which have not been observed in high pressure experiments. Comparison
of the phonon dispersions at 0 and 10 GPa reveals that softening of phonon
modes also occurs for the zone-boundary modes. The B1u and B3u modes play an
important role in transformation from the alpha-PbO2-type phase to baddeleyite
phase. The significant relaxations of the oxygen atoms from the Ti4 plane in
the Ti2O2Ti2 complex of the baddeleyite phase are directly correlated to the
oxygen displacements along the directions given by the eigenvectors of the soft
B1u and B3u modes in the alpha-PbO2-type phase.Comment: 8 pages, 9 figure
Large Electronic Anisotropy and Enhanced Chemical Activity of Highly Rippled Phosphorene
We investigate the electronic structure and chemical activity of rippled
phosphorene induced by large compressive strains via first-principles
calculation. It is found that phosphorene is extraordinarily bendable, enabling
the accommodation of ripples with large curvatures. Such highly rippled
phosphorene shows a strong anisotropy in electronic properties. For ripples
along the armchair direction, the band gap changes from 0.84 to 0.51 eV for the
compressive strain up to -20% and further compression shows no significant
effect, for ripples along the zigzag direction, semiconductor to metal
transition occurs. Within the rippled phosphorene, the local electronic
properties, such as the modulated band gap and the alignments of frontier
orbitals, are found to be highly spatially dependent, which may be used for
modulating the injection and confinement of carriers for optical and
photovoltaic applications. The examination of the interaction of a physisorbed
NO molecule with the rippled phosphorene under different compressive strains
shows that the chemical activities of the phosphorene are significantly
enhanced at the top and bottom peaks of the ripples, indicated by the enhanced
adsorption and charge transfer between them. All these features can be ascribed
to the effect of curvatures, which modifies the orbital coupling between atoms
at the ripple peaks
First-principles study of vibrational and dielectric properties of {\beta}-Si3N4
First-principles calculations have been conducted to study the structural,
vibrational and dielectric properties of {\beta}-Si3N4. Calculations of the
zone-center optical-mode frequencies (including LO-TO splittings), Born
effective charge tensors for each atom, dielectric constants, using density
functional perturbation theory, are reported. The fully relaxed structural
parameters are found to be in good agreement with experimental data. All optic
modes are identified and agreement of theory with experiment is excellent. The
static dielectric tensor is decomposed into contributions arising from
individual infrared-active phonon modes. It is found that high-frequency modes
mainly contribute to the lattice dielectric constant.Comment: 15pages, 1 figure, 5 table
Metformin-mediated increase in DICER1 regulates microRNA expression and cellular senescence
Metformin, an oral hypoglycemic agent, has been used for
decades to treat type 2 diabetes mellitus. Recent studies indicate
that mice treated with metformin live longer and have fewer
manifestations of age-related chronic disease. However, the
molecular mechanisms underlying this phenotype are unknown.
Here, we show that metformin treatment increases the levels of
the microRNA-processing protein DICER1 in mice and in humans
with diabetes mellitus. Our results indicate that metformin
upregulates DICER1 through a post-transcriptional mechanism
involving the RNA-binding protein AUF1. Treatment with metformin
altered the subcellular localization of AUF1, disrupting its
interaction with DICER1 mRNA and rendering DICER1 mRNA
stable, allowing DICER1 to accumulate. Consistent with the role
of DICER1 in the biogenesis of microRNAs, we found differential
patterns of microRNA expression in mice treated with metformin
or caloric restriction, two proven life-extending interventions.
Interestingly, several microRNAs previously associated with
senescence and aging, including miR-20a, miR-34a, miR-130a,
miR-106b, miR-125, and let-7c, were found elevated. In agreement
with these findings, treatment with metformin decreased
cellular senescence in several senescence models in a DICER1-
dependent manner. Metformin lowered p16 and p21 protein
levels and the abundance of inflammatory cytokines and oncogenes
that are hallmarks of the senescence-associated secretory
phenotype (SASP). These data lead us to hypothesize that
changes in DICER1 levels may be important for organismal aging
and to propose that interventions that upregulate DICER1
expression (e.g., metformin) may offer new pharmacotherapeutic
approaches for age-related disease
Surgical techniques and results of the pulmonary artery reconstruction for patients with central non-small cell lung cancer
Experimental Passage of St. Louis Encephalitis Virus In Vivo in Mosquitoes and Chickens Reveals Evolutionarily Significant Virus Characteristics
St. Louis encephalitis virus (SLEV; Flaviviridae, flavivirus) was the major cause of epidemic flaviviral encephalitis in the U.S. prior to the introduction of West Nile virus (WNV) in 1999. However, outbreaks of SLEV have been significantly more limited then WNV in terms of levels of activity and geographic dispersal. One possible explanation for these variable levels of activity is that differences in the potential for each virus to adapt to its host cycle exist. The need for arboviruses to replicate in disparate hosts is thought to result in constraints on both evolution and host-specific adaptation. If cycling is the cause of genetic stability observed in nature and arboviruses lack host specialization, then sequential passage should result in both the accumulation of mutations and specialized viruses better suited for replication in that host. Previous studies suggest that WNV and SLEV differ in capacity for both genetic change and host specialization, and in the costs each accrues from specializing. In an attempt to clarify how selective pressures contribute to epidemiological patterns of WNV and SLEV, we evaluated mutant spectra size, consensus genetic change, and phenotypic changes for SLEV in vivo following 20 sequential passages via inoculation in either Culex pipiens mosquitoes or chickens. Results demonstrate that the capacity for genetic change is large for SLEV and that the size of the mutant spectrum is host-dependent using our passage methodology. Despite this, a general lack of consensus change resulted from passage in either host, a result that contrasts with the idea that constraints on evolution in nature result from host cycling alone. Results also suggest that a high level of adaptation to both hosts already exists, despite host cycling. A strain significantly more infectious in chickens did emerge from one lineage of chicken passage, yet other lineages and all mosquito passage strains did not display measurable host-specific fitness gains. In addition, increased infectivity in chickens did not decrease infectivity in mosquitoes, which further contrasts the concept of fitness trade-offs for arboviruses
Dual modifications of α-Galactosylceramide Synergize to promote activation of human invariant natural killer T cells and stimulate anti-tumor immunity
Glycosylceramides that activate CD1d-restricted invariant natural killer T (iNKT) cells have potential therapeutic applications for augmenting immune responses against cancer and infections. Previous studies using mouse models identified sphinganine variants of α-galactosylceramide as promising iNKT cell activators that stimulate cytokine responses with a strongly proinflammatory bias. However, the activities of sphinganine variants in mice have generally not translated well to studies of human iNKT cell responses. Here, we show that strongly proinflammatory and anti-tumor iNKT cell responses were achieved in mice by a variant of α-galactosylceramide that combines a sphinganine base with a hydrocinnamoyl ester on C6″ of the sugar. Importantly, the activities observed with this variant were largely preserved for human iNKT cell responses. Structural and in silico modeling studies provided a mechanistic basis for these findings and suggested basic principles for capturing useful properties of sphinganine analogs of synthetic iNKT cell activators in the design of immunotherapeutic agents
Claudin-7 Is Frequently Overexpressed in Ovarian Cancer and Promotes Invasion
Background: Claudins are tight junction proteins that are involved in tight junction formation and function. Previous studies have shown that claudin-7 is frequently upregulated in epithelial ovarian cancer (EOC) along with claudin-3 and claudin-4. Here, we investigate in detail the expression patterns of claudin-7, as well as its possible functions in EOC. Methodology/Principal Findings: A total of 95 ovarian tissue samples (7 normal ovarian tissues, 65 serous carcinomas, 11 clear cell carcinomas, 8 endometrioid carcinomas and 4 mucinous carcinomas) were studied for claudin-7 expression. In real-time RT-PCR analysis, the gene for claudin-7, CLDN7, was found to be upregulated in all the tumor tissue samples studied. Similarly, immunohistochemical analysis and western blotting showed that claudin-7 protein was significantly overexpressed in the vast majority of EOCs. Small interfering RNA-mediated knockdown of claudin-7 in ovarian cancer cells led to significant changes in gene expression as measured by microarrays and validated by RT-PCR and immunoblotting. Analyses of the genes differentially expressed revealed that the genes altered in response to claudin-7 knockdown were associated with pathways implicated in various molecular and cellular functions such as cell cycle, cellular growth and proliferation, cell death, development, and cell movement. Through functional experiments in vitro, we found that both migration and invasion were altered in cells where CLDN7 had been knocked down or overexpressed. Interestingly, claudin-7 expression was associated with a net increase in invasion, but also with a decrease in migration
Induction of seed germination in Orobanche spp. by extracts of traditional Chinese medicinal herbs
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